1. Technical Field of the Invention
The present invention relates to a method of calculating the points and normal lines of contact of the tools or parts on a processing or assembly apparatus for use in such broad applications as repetitive work requiring processing along the surface of the product, or assembly work requiring contact between parts. The present invention also pertains to a control device for controlling an apparatus for processing or assembly.
2. Related Art
Concerning apparatuses in the prior art for determining the contact points between the tools/parts of processing/assembly apparatuses and the products being processed/assembled, especially in the case of robotic arms, examples are reported in the following documents.
Izumi, et al., disclose a contact point determination method which uses reaction force information in "The Japanese Robotics Society Journal", vol. 4, no. 2, pages 27 through 34. Izumi's method enables detection of contact points based on a limited number of force components by modeling friction forces. However, the method assumes that the friction coefficient between the tools and work piece or the friction force between parts is known. Therefore, Izumi's method is not applicable to the situations wherein the friction coefficient is not known or the friction coefficient varies during processing or assembly operations. Furthermore, since the error in the force measurement is supposed to be negligible in Izumi's method, contact points thus detected tend to contain error which is originated from the error in the force measurement.
Kitagaki, et al., disclose a method of determining the contact point based on the deviation force vector calculated when the load weight is changed by controlling the orientation without changing the contact point in "The Japanese Robotics Society 8th Convention Abstract (1)," page 395 through 398. According to Kitagaki's method, contact points are detected without referring to the pre-inputted data regarding configuration of the tool. However, the method has a shortcoming that the operation such as grinding etc., must be stopped while the orientation of the tool is being changed and force data is being obtained. Therefore, the method is not suitable for repetitive operations wherein the orientation of the tool is predetermined or not suitable for the operations wherein the contact point varies when the orientation of the tool is changed.
A method for determining the normal line of contact using the three force components from a force measurement device is reported in Japanese Patent Application First Publication No. Hei 3-256103. The method disclosed therein is not suitable for the situations wherein the friction force between the tool and workpiece or between parts are not negligible or the situations wherein contact points vary because the contact points between the tool and the work piece are supposed not to change during operation.
In the technologies described above, an additional problem is the error in force measurement derived from the weight or acceleration of the tool and parts. Even though efforts are made to reduce the measurement error by considering the weight, gravity center, inertial moment, geometry of the robot arm, the error in force measurement is not effectively reduced because of the difficulty in obtaining information regarding the parameters described above.
Furthermore, it is crucial that the technologies described above are not essentially capable of eliminating error in the force measurement which might be caused due to various sources such as temperature variation, hysteresis, non-linearity in various phenomena.
A contact point determination technology described on page 127 of "The Japanese Robotics Society Journal", vol. 11, no. 3 is a method of calculating the normal lines and points of contact in which a determination function is calculated from the distance between the surface which the force application line crosses and chosen points located on the tool surface, and the dot product of the translation velocity and the normal vector, and the contact point and contact normal line are determined by the chosen point which minimizes the determination function. This method is applicable to situations wherein friction force varies and the contact points varies during operation because the geometry of tool is pre-inputted, and the contact point is determined as the cross point of the force vector and the surface of the tool defined by the pre-inputted geometrical data. The measurement errors due to weight of the tool and work pieces, variation of the temperature, hysteresis, non-linearity can be taken into account. However, the method is less effective with regard to the suppression of the error component in the direction perpendicular to the direction of translation of the tool.
In the unknown boundary estimation method given on pages 289 and 290 of "The Abstract of the 1989 Measurement Automatic Control Science Convention", a boundary direction estimation method is demonstrated which takes the effect of friction into consideration by removing the velocity component from the direction of the force components, but in this method, the contact point between the end effector and the object being worked on must always be fixed during the task specifications, or the translation velocity of an arbitrary point on the end effector must be fixed, meaning that the area of possible application only includes work which does not include changes in the orientation during the procedure, and there is a problem in that the application to cases in which the contact point or orientation changes is difficult.
A. Bicchi et al., disclose a method for detecting a contact point wherein the friction force component in the direction perpendicular to the direction of contact is taken into consideration instead of assuming that the crossing point of the direction of force with the surface of the object is the contact point, in "Contact Sensing from Force Measurements", The International Journal of Robotics Research, Vol. 12, No. 3, Massachusetts Institute of Technology, June 1993. Even though the method employs all the possible friction components, the method is not effective in suppressing the error in force measurement because the method assumes that the measurement of force components are exact.